REVERSED AND NON-REVERSED SPECTRA. 



143 



it was useful for testing the ring method in comparison with the former. The 

 equivalent of a vanishing interference ring is here not immediately given in 

 terms of the wave-length of light, since the rings move through the spectrum. 



With the exception of a few incidental experiments of my own, optic 

 methods of the present kind have not hitherto been used. They are here par- 

 ticularly applicable, since the number of the rings vanishing in a given region 

 of the spectrum has merely to be counted after the sudden exhaustion and 

 during the period of slow influx of air. 



Succeeding parts of the chapter will refer to other available forms of ap- 

 paratus with similar ends in view, and the additional purpose of ascertaining 

 how long and narrow an apparatus may be shaped, without seriously inter- 

 fering with the adiabatic measurements; for if the apparatus is increased 

 indefinitely in length and diameter, it is obvious that the suddenness of the 

 exhaustion through any available pipe will be more and more impaired. The 

 same is true if the apparatus, for a given (sufficient) length, is too narrow, 

 though for a different reason. 



TABLE 17. Values of y. Bulky air chamber, 7=99,800 cub. cm., v= 11,620 cub. cm. 

 (F+tO/F=i.ii6. C=952.6; 1+*= 1.0341; 0=20.3 cm - 



1 Count broken owing to flicker of arc; obtained from rhythm. 2 Sunlight. 



87. Experiments with short, bulky air=chambers. An example of the data 

 obtained is given in table 17, where the ratio of specific heats, 7, computed 

 directly both from displacement of ellipses and 7' from interference rings, is 

 shown in detail. The original pressure of the air-chamber is that of the barom- 

 eter, pQ. The pressure of the vacuum chamber is given under p. The dis- 

 placement, AN, from four independent observations in each case and the 

 number of interference rings vanishing from exhaustion to plenum are the 

 data chiefly of interest. It has not been possible, according to the table, to 

 *Carnegie Inst. Wash. Pub. 149, Part II, 83, p.129; 85, p. 135. 1912. 



